1. Field of the Invention
The present invention relates to an ink jet recording ink, an ink jet recording ink set, and an ink jet recording method and apparatus using the ink and the ink set.
2. Description of the Related Art
The principle of ink jet recording systems is that liquid or melted solid ink is jetted out through nozzles, slits, porous films or the like onto recording media such as paper, cloth, film and the like to thereby record images and others on the media. These recording systems provide various advantages in that they are small in size, inexpensive in price, and quiet in operation, and they have come onto the market as a monochrome printer that forms black images and a color printer that provides full-color images. Among these, a so-called piezoelectric ink jet recording system using a piezoelectric element, or a so-called thermal ink jet recording system in which liquid droplets are formed by applying thermal energy for recording has many advantages such as high speed printing, high resolution printing and the like.
The ink used in the ink jet recording apparatuses must be able to achieve the following:    (1) provide images having high resolution, high density and uniformity, without bleeding or fogging on paper;    (2) cause no clogging at the tip of a nozzle due to dried ink, and exhibiting good ejecting response and good ejecting stability;    (3) provide excellent drying characteristics on paper.    (4) provide good fastness of images; and    (5) provide good long-term storage stability.Various proposals have been made to satisfy the above demands.
Conventionally, of those types of ink, aqueous ink that contains a water-soluble dye is the mainstream of ink jet recording ink. Since, however, such a dye is soluble in water, problems of water resistance and light fastness occur, and an aqueous ink does not necessarily provide satisfactory stability in the storage of images. On the other hand, inks using a pigment as a color material offer promising prospects insofar that both water resistance and light fastness can be improved. Various proposals have therefore been made with regard to inks containing a pigment, and several have been put into practical use.
However, while a pigment ink does produce an improvement in terms of fastness to water, it also has a drawback insofar that it tends to aggregate if it is stored for a long period of time, or at a high temperature, and storage stability of the pigment ink thus becomes a problem. Further, optical density and dispersion stability have a diametrically-opposed relationship, caused by the strength of the cohesive force of pigments. Although a high optical density can be obtained as a result of the aggregation of pigments on paper after printing, the strong cohesive force also results in a deterioration in dispersion stability.
Among attempts to achieve both dispersion stability and high optical density in an aqueous pigment ink for ink jet recording, a technique has been proposed that sets a range based on a relationship between an average particle size of a pigment and a blackness index (for example, see JP-A No. 11-80636). However, the range specified in that application makes it difficult to use effectively a pigment whose average particle size is relatively large. Also, for the same purpose, a method has been proposed of using two or more types of black ink at the same time (see JP-A No. 2000-7964). However, this method also has a drawback insofar that, although it does produce an improvement in optical density, the structure becomes complicated because two or more types of black ink are used at the same time.
Among methods adopted to obtain high optical density, have been: (i) increasing the density of a pigment in an ink; (ii) increasing the hydrophobic property of a dispersing agent; (iii) reducing an amount of dispersing agent added; (iv) increasing an amount of oil absorption in a pigment; and the like. However, in method (i), the storage stability of the ink deteriorates because of the aggregation of pigments, and clogging of the ink head has also tended to deteriorate. Further, all of methods (ii) to (iv) have drawbacks insofar that storage stability, particularly, storage stability at a high temperature, tends to deteriorate, and in some instances, this tendency is pronounced. For this reason, in order to secure dispersion stability, it has been necessary to adopt a method with conditions opposite to those required for increasing optical density, and up to the present, high optical density and dispersion stability have not been achieved simultaneously.
Accordingly, the present invention has been made to provide: a pigment-dispersion ink jet recording ink, which secures a high optical density while dispersion stability of the pigment is maintained, which does not cause clogging in either an ink flow channel or in a nozzle portion at the time of printing, and which also has excellent printing qualities such as fixing strength, water resistance, and light fastness; an ink set that includes such an ink; and an ink jet recording method and apparatus using the ink and the ink set.